The wastewater sludge used in this invention is a byproduct of paper recycling plants that produce recycled pulp. The waste sludge contains water, delignified cellulose debris, clay from paper coatings, dirt and other undesirable constituents. Until this invention, the sludge had no practical use; it had to be disposed of in land fills as waste. Recycling plants are producing well over 400 tons of the sludge each day.
Although local regulations and conditions vary between recycling plants, the trend is very clear that current disposal methods and landfill use will become more restricted and costly.
Today, there is an ever increasing emphasis on resource recovery and recycling. As a consequence, it is more and more necessary to capture or recover all cellulose fiber material that can be used in paper manufacture and to improve the yield and quality of the recovered secondary fibers. Not all of the cellulose fiber material can be used in the manufacture of paper. The unusable cellulosic debris is fractured, broken, frayed and split into very small particles. The cellulose debris particles have an aspect ratio (length to width ratio) generally less than 100-to-1 and are shorter than 0.5 cm i.e., 50 mm. The ASTM definition of a fiber does not include this material because the definition requires a greater length and greater aspect ratio; according to the ASTM definition a fiber has a length of at least 0.5cm and an aspect ratio of 100-to-1 or greater. Accordingly, cellulose debris from paper recycling is not fibrous despite the fact that the material recovered in such recycling is fibrous.
The wastewater sludge from fiber recovery contains three to six percent solids by weight. For waste disposal, the sludge is dewatered to 25-40 percent solids.
A typical recycling plant mechanically de-inks and repulps waste paper. The waste paper raw material is primarily bond, ledger, and color ledger materials. After de-inking and repulping, the pulp is bleached, yielding a clean, bright, recycled pulp. Bleaching in the paper industry is any process where residual lignin or hemicellulose or color bodies are removed by any means from the cellulose fibers. Bleaching can be accomplished chemically or mechanically and chemically.
Wastewater treatment recycles much of the process water back to the recovery process. Wastewater treatment typically includes chemical and physical treatment, secondary biological treatment and secondary clarification. In addition to solids removed in the wastewater treatment process, there is a solids removal step internal to de-inking that produces more waste; the waste is combined with waste from wastewater treatment to produce the three to six percent solid wastewater sludge. This sludge has polyelectrolytes added and is dewatered to a solids content of 25 to 40 percent by weight. This sludge has a high biological activity. The solid matter in the sludge is about 55 percent cellulose debris, 45 percent kaolinitic clay, including one percent other impurities. The cellulose debris has been stripped of lignin, leaving hollow tubes that collapse into long, flat ribbons. The individual particles of the debris are also severely splintered or fibrillated along their lengths. As previously pointed out, the particle size of the cellulosic debris is less than 0.5 cm with an aspect ratio of less than 100-to-1.
Organic fibers have been used to reinforce building compositions since prehistoric clay bricks were reinforced with straw. For example, U.S. Pat. No. 4,402,751 to Wilde discloses a method for using waste cellulose fibers such as wood pulp, rag pulp and linen pulp from sludge to form a cementitious composition. Wilde uses lime to prevent biological decomposition of cellulose. The lime does not delignify the cellulose; it simply encapsulates it.